This invention relates generally to a heat exchange device, or a thermosyphon. It relates, more particularly, to a two-phase thermosyphon for removing heat from a permafrost foundation with an evaporator which may have both positively and negatively sloped, or rising and falling, sections.
It has been known to use a gravity-controlled two-phase thermosyphon for maintaining a permafrost foundation system by extracting heat from the ground and radiating it out to the atmosphere whenever the atmospheric temperature is below that surrounding its evaporator. Such a thermosyphon operates on the physical principles that (1) a vapor of any substance is lighter than its liquid state, (2) the vapor pressure is generally increased with an increase in temperature, and (3) the vapor density at a constant volume decreases with a reduction in temperature. E. L. Long U.S. Pat. No. 3,217,791, issued Nov. 16, 1965, discloses a thermosyphon which operates on these principles, comprising a sealed container partially filled with a liquid having a low boiling point. Vapor from this liquid is caused to rise and condense in a vertically elongated upper heat-dissipating portion of The container when the temperature is below the freezing point, thereby lowering the vapor pressure inside the container. This causes boiling of the liquid in the lower portion of the container with a consequent reduction in temperature, and heat flows from the surrounding permafrost to the container.
One of the requirements for such a heat exchange device to work as described above is that the condensate liquid be able to flow the full length of the lower liquid-evaporating portion of the container. In other words, it is necessary to provide a wetted surface along the full length of the evaporator. In the case of a very long evaporator, however, this means an increased depth of burial within the permafrost, and such burial adds great expense to the overall cost of the installation.
For many applications, on the other hand, it is preferable to install the evaporator of such a thermosyphon substantially horizontally, say, on a level surface. DenHartog, et al. U.S. Pat. No. 4,961,463, issued Oct. 9, 1990, discloses a thermosyphon with a condensate return tube running along the entire length of the evaporator to its distal end such that heat can be removed from a permafrost foundation or the like even when its evaporator section is disposed horizontally or at a "negative slope" (that is, sloping upwards as one moves away from the condenser section). This device, however, cannot be operated on a "positive slope" (which rises as one moves away from the condenser section) except from an overflow portion, if any, of the liquid from its condensate-collecting device. In other words, great care must be taken in the installation of such a device in order to make certain that its evaporator should not only be horizontal or negatively sloped when it is first installed but also remain so in spite of the differential settlement and/or heave that may occur after installation and prior to or after beginning of its operation. If rising and falling sections develop in such a prior art evaporator, liquid condensate may not be able to move beyond the first slope-reversing point from each end, and little or no cooling may take place over the majority of the evaporator section. Where there are existing obstructions, furthermore, it may be desirable, or even imperative, to install an evaporator with rising and falling sections to bypass such obstructions.